The Dimensionizer is a contextual menu plugin for Mac OS X that allows you to discover the dimensions (height and width) of virtually any image with a single click.
Best of all, you don’t even have to remember the dimensions to use them. Just select the menu item representing an image, and Dimensionizer will copy the image dimensions to the clipboard.
System Requirements: Mac OS X 10.3 or newer.
This should be very useful.